The present invention relates to a magnetic resonance imaging (MRI) system and, more particularly, to an MRI system which can eliminate or remove degradation in image quality caused by a respiratory movement, cardiac cycles, and other body movement of a subject under medical examination; for example, degradation in image quality caused by a ghost-like artifact.
As is already known, the magnetic resonance imaging (MRI) technique utilizes a magnetic resonance phenomenon to obtain chemical and physical microscopic information of molecules. Using this phenomenon, the energy of an RF magnetic field, which is rotated at a specific frequency in a plane perpendicular to the direction of a static magnetic field, is resonantly absorbed when a group of nuclear spins unique to specific atomic nuclei, which have magnetic moments, is placed in a uniform static magnetic field.
By using the MRI technique, the spatial distribution of specific atomic nuclei, for example, hydrogen atomic nuclei (contained in water and fat), can be visualized. As examples of MRI methods, a projection reconstruction method by Lauterbur, a Fourier transformation (FT) method by Kumar, Welti, Ernst, a spin warp method by Hutchison et al. as a modification of the FT method, an echo planar method by Mansfield, and the like are known.
Some MRI systems are capable of performing high-speed imaging, (for example, a system using the echo planar method) wherein all the image-reconstruction data can be acquired within several tens of milliseconds. However, in MRI systems other than those which can perform such high-speed imaging, the data acquisition time is long, in the order of minutes. For this reason, degradation in image quality caused by body movements, for example, degradation in image quality due to a ghost-like artifact and/or blurring, caused by body movement, tends to occur with respect to images of internal organs other than regions such as the central nervous system, the pelvic region, the legs and arms, and the like, which are less influenced by body movements. In particular, degradation in image quality tends to occur with respect to images of such regions as the thorax and the abdominal region which are influenced considerably by body movement such as respiratory movement and/or cardiac cycles. Such degradation in image quality is caused since the phase and amplitude of image reconstruction data (which is obtained by detecting a magnetic resonance (MR) signal) cause semi-periodical variations in the direction of the encoding axis (of phase-encoding for providing positional information to the MR signal) due to semi-periodical body movement such as respiratory movement and/or cardiac cycles.
Known methods for suppressing such degradation in image quality include (1) a method wherein a subject being examined is fastened by a band so as to forcibly suppress body movement, (2) a method wherein respiratory movement is monitored, and expansion and contraction caused by respiration are corrected by controlling the intensity of a gradient field or by software processing, (3) a method wherein an encoding order is reordered to differ from that of the normal method, so as to apparently delay the body movement cycle, and the like.
Method (1) is effective in some cases. However, it applies a physical load to the subject, and causes pain and discomfort to the subject (i.e., the patient). In methods (2) and (3), respiratory movement must be monitored, the intensity of the gradient field must be controlled, and complex processing of software is required. The system using method (2) or (3) is therefore complicated, and is not versatile. Method (2) is based on the assumption that body movement, i.e., the extraction and contraction, occurs linearly in the direction of a gradient field used for encoding or reading data. The actual body movement is much different from such linear movement. In view of this, method (2) can be said to be imcomplete.
An artifact caused by blood flow can be considered as one of those caused by body movement. None of methods (1) to (3) are effective to counter the effects of this particular body movement.
As has been described above, in MRI systems using the conventional FT or spin warp method, it is difficult to easily and effectively eliminate degradation in image quality caused by the body movement of a subject under examination.